1. Field of the Invention
The present invention relates to a printing apparatus and a processing method therefor.
2. Description of the Related Art
A printing apparatus which prints on a printing medium is known. When the printing apparatus receives data from, e.g., a host, it rasterizes the received data into binary bitmap data. The printing apparatus then transfers the bitmap data to a printhead to print it. Binary bitmap data or multi-valued data is sent from the host to the printing apparatus. If the sent data is multi-valued data, the printing apparatus rasterizes it. The rasterization timing is, for example, the time of H/V conversion of data when it is stored in a buffer or the time of sending data to the printhead (printhead control unit) when multi-valued data is stored in the buffer intact.
If a fixed rasterization table (dot matrix) alone is set for the tone of each pixel in preparation for the above-mentioned rasterization, a single rasterization pattern is obtained for the same tone. Such rasterization into a fixed pattern may generate stripes and nonuniformity attributed to nozzle contamination or a variation in nozzle discharge amount.
To combat this situation, one known method sets a plurality of dot matrices for the tone of each pixel, selects one of these plurality of dot matrices, and rasterizes the selected dot matrix. In this case, a pattern corresponding to each tone is not fixed and therefore the printing result is less subject to, e.g., nozzle contamination.
Some methods of selecting one of a plurality of dot matrices have been proposed. Examples of these methods are a method of selection based on the column position, a method of random selection by generating random numbers, and a method of changing the dot matrix for use in rasterization of a given tone every time data with the given tone appears (Japanese Patent Laid-Open Nos. 2000-141617 and 2004-209765).
In the method of selection by changing the dot matrix for use in rasterization of a given tone every time data with the given tone appears, it is often the case that the results of printing the same data in only the forward direction and in both the forward and backward directions are inconsistent with each other.
This inconsistency between these printing results will be explained herein. To rasterize a raster in the forward direction in forward printing, the initial value of the pattern number is set for the raster leading edge, dot matrices are sequentially selected based on the set initial value, and respective data within the raster are rasterized using the selected matrices. In contrast, to rasterize a raster in the backward direction in backward printing, the initial value of the pattern number is set for the raster trailing edge, dot matrices are sequentially selected based on the set initial value, and respective data within the raster are rasterized using the selected matrices. As a result, even data with the same tone at the same position is rasterized in a pattern which differs between when rasterization starts from the raster leading edge and when rasterization starts from the raster trailing edge. In other words, when the matrix is changed every time data appears in rasterization in the forward raster direction (forward direction), the pattern at the raster trailing edge, which has an initial value in rasterization in the backward raster direction (backward direction), differs depending on the number of tones within the raster.